Abstract
Insect herbivores can shift the composition of a plant community, but the mechanism underlying such shifts remains largely unexplored. A possibility is that insects alter the competitive symmetry between plant species. The effect of herbivory on competition likely depends on whether the plants are subjected to aboveground or belowground herbivory or both, and also depends on soil nitrogen levels. It is unclear how these biotic and abiotic factors interactively affect competition. In a greenhouse experiment, we measured competition between two coexisting grass species that respond differently to nitrogen deposition: Dactylis glomerata L., which is competitively favoured by nitrogen addition, and Festuca rubra L., which is competitively favoured on nitrogen-poor soils. We predicted: (1) that aboveground herbivory would reduce competitive asymmetry at high soil nitrogen by reducing the competitive advantage of D. glomerata; and (2), that belowground herbivory would relax competition at low soil nitrogen, by reducing the competitive advantage of F. rubra. Aboveground herbivory caused a 46% decrease in the competitive ability of F. rubra, and a 23% increase in that of D. glomerata, thus increasing competitive asymmetry, independently of soil nitrogen level. Belowground herbivory did not affect competitive symmetry, but the combined influence of above- and belowground herbivory was weaker than predicted from their individual effects. Belowground herbivory thus mitigated the increased competitive asymmetry caused by aboveground herbivory. D. glomerata remained competitively dominant after the cessation of aboveground herbivory, showing that the influence of herbivory continued beyond the feeding period. We showed that insect herbivory can strongly influence plant competitive interactions. In our experimental plant community, aboveground insect herbivory increased the risk of competitive exclusion of F. rubra. Belowground herbivory appeared to mitigate the influence of aboveground herbivory, and this mechanism may play a role for plant species coexistence.
Highlights
Herbivory and soil nutrient availability are two factors that strongly influence competitive relationships between plant species (Keddy, 2007)
The competitive interaction between D. glomerata and F. rubra, as measured by the aggressivity index, was altered by herbivory (Fig. 1A and Table 1), such that D. glomerata benefited from the presence of herbivores
There was a non-additive effect of aboveand belowground herbivory on aggressivity when applied together, such that the effect of combined above- and belowground herbivory was smaller than would be expected from the individual effects of each herbivory treatment (Fig. 1A and Table 1)
Summary
Herbivory and soil nutrient availability are two factors that strongly influence competitive relationships between plant species (Keddy, 2007). There is evidence from field experiments demonstrating shifts in community composition under insect herbivory (Brown & Gange, 1989; Carson & Root, 2000; Van Ruijven et al, 2005; Allan & Crawley, 2011). Theory suggests that such community-level shifts are driven by changes in interspecific competition among plants, as even low rates of biomass removal by insects can alter the competitive hierarchy of a plant community (Louda, Keeler & Holt, 1990). There is, little experimental evidence detailing to what degree, and under which environmental circumstances, insect herbivory mediates plant competition (Sotomayor & Lortie, 2015)
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